This application claims the benefit of Japanese Patent application No. 2000-137731 which is hereby incorporated by reference.
1. Field of the Invention
This invention relates to a progressive-power multifocal lens, and more particularly to a progressive-power multifocal lens used to assist the ability of accommodation of the eye.
2. Related Background Art
Single-vision lenses, bifocal lenses or progressive-power multifocal lenses are used to correct presbyopia. Of these lenses, in particular, the progressive-power multifocal lenses make it unnecessary to put on and put off spectacles between those for far vision and those for near vision, and also have an external appearance having no boundary line like that of bifocal lenses. Accordingly, in recent years, there is a reasonably increasing demand for the progressive-power multifocal lenses.
The progressive-power multifocal lenses are spectacle lenses for assisting the ability of accommodation of the eye when it grows weak to make the near vision difficult. The progressive-power multifocal lenses commonly have a far-vision correction portion and a near-vision correction portion which are positioned at the upper part and the lower part, respectively, of a lens at wear of spectacles (hereinafter, the former is often xe2x80x9cfar-vision portionxe2x80x9d and the latter often xe2x80x9cnear-vision portionxe2x80x9d), and a progressive-power portion where the refracting power changes continuously, positioned between the both portions (hereinafter often xe2x80x9cintermediate portionxe2x80x9d). Incidentally, in the present specification, terms xe2x80x9cupper partxe2x80x9d, xe2x80x9clower partxe2x80x9d, xe2x80x9chorizontalxe2x80x9d and xe2x80x9cverticalxe2x80x9d describe the positional relationship of a lens at wear of spectacles. For example, the lower part of the far-vision portion is meant to be the portion close to the intermediate portion within the region of the far-vision portion.
FIG. 1 illustrates an outline of regional sections of a progressive-power multifocal lens which are designed symmetrically. The progressive-power multifocal lens shown in FIG. 1 has a far-vision portion F and a near-vision portion N which are positioned at the upper part and the lower part, respectively, of the lens at wear of spectacles, and an intermediate portion P where the refracting power changes continuously, positioned between the both portions. With regard to the shape of lens surface, a line of intersection M-Mxe2x80x2 where a cross section along the meridian which runs vertically through substantially the middle of lens surface from the upper part to the lower part intersects with the object-side (the side opposite to the eye) lens surface is used as a reference line for representing specification such as additional power of lenses. In the progressive-power multifocal lenses thus designed symmetrically, the center of far-vision portion, OF, of the far-vision portion F, the far-vision eyepoint E which is a fitting point, the geometric center of lens surface, OG, and the center of near-vision portion, ON, are positioned on the center line M-Mxe2x80x2 serving as a reference.
FIG. 2 illustrates an outline of regional sections of a progressive-power multifocal lens which are designed asymmetrically, taking account of the fact that the center of near-vision portion, ON, of a lens comes close to the nasal side in wear of spectacles (this lens is hereinafter xe2x80x9casymmetrical progressive-power multifocal lensxe2x80x9d). In the asymmetrical progressive-power multifocal lens shown in FIG. 2, too, the center line M-Mxe2x80x2 consisting of a line of intersection where a cross section which passes along the center of far-vision portion, OF, of the far-vision portion F, the far-vision eyepoint E, the geometric center of lens surface, OG, and the center of near-vision portion, ON, intersects with the object-side lens surface is used as a reference line.
In the present specification, these reference lines are generically called xe2x80x9cprincipal meridian curvexe2x80x9d. The center of the far-vision portion F and the center of the near-vision portion are positions used as references when lens dioptric power is measured. A distance (far-vision) portion measurement reference point is called the center of far-vision portion, OF, and a near-portion measurement reference point is called the center of near-vision portion, ON. Also, mean surface refracting power at the the center of far-vision portion, OF, is regarded as a base curve, and mean dioptric power in respect of a light ray which passes through the center of far-vision portion, OF, is regarded as reference mean dioptric power at the far-vision portion (hereinafter xe2x80x9cfar-vision portion dioptric power). Usually, the center of near-vision portion, ON, is in agreement with the near-vision eyepoint. Note, however, that the center of far-vision portion and the center of near-vision portion which are herein referred to are meant not to be geometric centers in the respective portions but to be functional centers in the measurement of lenses and at wear of spectacles.
In the present specification, mean surface refracting power (hereinafter often xe2x80x9csurface refracting powerxe2x80x9d) and surface astigmatism are expressed by the following equations (a) and (b), respectively, where a maximum main curvature at an arbitrary point on the progressive-power multifocal surface is represented by "psgr"max, a minimum main curvature by "psgr"min, and a refractive index of a lens by n.
Surface refracting power=("psgr"max+"psgr"min)xc3x97(nxe2x88x921)/2 xe2x80x83xe2x80x83(a) 
Surface astigmatism=("psgr"maxxe2x88x92"psgr"min)xc3x97(nxe2x88x921) xe2x80x83xe2x80x83(b) 
In the present specification, mean dioptric power and astigmatism are expressed by the following equations (c) and (d), respectively, where maximum dioptric power in respect of a light ray having passed through an arbitrary point on the progressive-power multifocal surface is represented by Dmax, and minimum dioptric power by Dmin.
Mean dioptric power=(Dmax+Dmin)/2 xe2x80x83xe2x80x83(c) 
Astigmatism=(Dmaxxe2x88x92Dmin) xe2x80x83xe2x80x83(d) 
In the present specification, mean surface additional refracting power (hereinafter often xe2x80x9csurface additional refracting powerxe2x80x9d) refers to surface refracting power found by subtracting the base curve from the surface refracting power at an arbitrary point on the progressive-power multifocal surface. Also, mean additional dioptric power (hereinafter often xe2x80x9cadditional dioptric powerxe2x80x9d) refers to dioptric power found by subtracting far-vision portion dioptric power from the mean dioptric power (hereinafter often xe2x80x9cdioptric powerxe2x80x9d) in respect of a light ray which passes through an arbitrary point on the progressive-power multifocal surface.
In the progressive-power multifocal lens, a plus surface refracting power (or dioptric power) is continuously added toward the center of near-vision portion, ON, from the center of far-vision portion, OF, on the principal meridian curve M-Mxe2x80x2 which passes substantially the geometric center of the lens. The value found by subtracting the surface refracting power (or dioptric power) of the center of far-vision portion, OF, from the surface refracting power (or dioptric power) of the center of near-vision portion, ON, at which this surface additional refracting power (or additional dioptric power) comes to be maximum is called additional power of the progressive-power multifocal lens. In the progressive-power multifocal lens, what is ideal is a lens having distinct vision in a wide range and less crooked or distorted view in all regions of the far-vision portion F, the intermediate portion P and the near-vision portion N.
Now, in conventional progressive-power multifocal lenses, discussion has been made chiefly on optical characteristics of the progressive-power multifocal surface (refracting surface). More specifically, the performance of progressive-power multifocal lenses has often been evaluated on, e.g., their distribution of surface refracting power (or distribution of surface additional refracting power) and distribution of astigmatism at the progressive-power multifocal surface. Accordingly, design engineers have chiefly aimed at attaining distribution of surface refracting power adapted to purposes, widely ensuring the portion having astigmatism not greater than a stated value, i.e., the region called a distinct-vision range and also making the maximum value of astigmatism as small as possible taking account of any deformed, crooked or distorted view, at the progressive-power multifocal surface.
In actual spectacle lenses, however, the optical characteristics of the progressive-power multifocal surfaces of lenses do not necessarily coincide with optical characteristics the lenses may have when those who wear spectacles use the lenses. Accordingly, in recent years, in order to more improve optical performance perceived when those who wear spectacles use lenses actually, it has come to make evaluation of not only optical characteristics of the progressive-power multifocal surface but also optical performance in a state closer to the state where spectacles are worn, i.e., to make evaluation of optical performance on the basis of light rays having passed through the lenses.
In general, the relationship between lens curvature and lens dioptric power that may make minimum the astigmatism of light rays having passed through a lens can be obtained from, e.g., Tscherning""s Ellipse. That is, it is well known that, as curvature of each surface of a lens, combination of optimum curvature obtained by the Tscherning""s Ellipse may be selected so that any astigmatism can be kept from occurring at the border of the lens. However, when any combination of optimum curvature obtained by the Tscherning""s Ellipse is used, the base curve may have so large curvature that the lens also tends to have a large thickness. Accordingly, in progressive-power multifocal lenses available in recent years, it has become prevailing that curvature which is smaller than the curvature obtained from the combination of optimum curvature is selected as the base curve.
Hence, between the distribution of surface refracting power or distribution of astigmatism at the progressive-power multifocal surface and the distribution of dioptric power or astigmatism in respect of light rays having passed through the lens to enter the eye of those who wear spectacles, what shows an equal tendency is limited, in many cases, to the portion where light rays coming from an object is incident at angles almost perpendicular to the lens surface, i.e., the portion of the optical axis of the lens and the vicinity thereof, such as the fitting point of the lens and the vicinity thereof. In contrast thereto, light rays which come through positions distant from the optical axis of the lens and enter the eye of those who wear spectacles is incident obliquely to the lens surface. Hence, it follows that light rays which pass through the lens at its positions where the surface refracting power of the lens surface is equal to the base curve and the astigmatism is substantially zero also stand deviated in dioptric power from the far-vision portion dioptric power serving as a reference when they pass through the lens, and enter the eye of those who wear spectacles, in the state where astigmatism has occurred. This tendency varies depending on curvature, center thickness and so forth of prescribed surfaces of the lens and moreover becomes greater at positions coming nearer to the border of the lens. Accordingly, it becomes necessary to design progressive-power multifocal surface which has been made optimum taking account of various conditions such as base curve, power and so forth of the lens.
Recently, in progressive-power multifocal lenses, techniques are proposed in which optical performance is evaluated on the basis of such light rays. In such prior art, however, a portion having an astigmatism not greater than a stated level, stated specifically, a region where the astigmatism is not greater than 0.50 diopter is defined to be a distinct-vision range, and discussion has almost been made only on how to ensure this distinct vision in a wide range. More specifically, in the prior art, discussion is little made on how to make optimum the distribution of dioptric power or distribution of additional dioptric power.
In order to widen the distinct-vision range in wear of spectacles, it is important and necessary to control the astigmatism to a low level. Especially with regard to the far-vision portion, it can not be said to be sufficient if the distinct-vision range is defined only by the level of astigmatism. More specifically, in a region where the dioptric power stands deviated greatly from any prescribed far-vision portion dioptric power, a view may blur because of a dioptric-power error even if the astigmatism is not greater than the stated level commonly defined to be the distinct-vision range (stated specifically, the region where the astigmatism is not greater than 0.50 diopter). Hence, those who wear spectacles can not distinctly see any objects at a long distance. The influence due to the dioptric-power error at the far-vision portion for seeing objects at a long distance is greater than any influence due to a dioptric-power error at the near-vision portion for seeing objects at a short distance. Accordingly, it is more greatly important for the far-vision portion to be designed taking account of any dioptric-power error, deviated from the stated far-vision portion dioptric power, than for the near-vision portion.
The present invention was made taking account of the above problems. Accordingly, an object of the present invention is to provide, in progressive-power multifocal lenses whose far-vision portion dioptric power is minus, a progressive-power multifocal lens in which the level of deviation of dioptric power, i.e., the dioptric-power error, from far-vision portion dioptric power at the far-vision portion can be controlled to a small value over a wide range and the optical performance in wear of spectacles can be set well.
Another object of the present invention is to provide a progressive-power multifocal lens that may less cause any blur of views due to the dioptric-power error and can widely ensure the distinct-vision range.
To achieve the above objects, the present invention provides a progressive-power multifocal lens whose far-vision portion dioptric power is minus, which comprises, along a principal meridian curve which divides the refracting surface of the lens into a nasal-side portion and a temporal-side portion;
a far-vision correction portion which deals with a long distance;
a near-vision correction portion which deals with a short distance; and
a progressive-power portion which connects the refracting power continuously between the far-vision correction portion and the near-vision correction portion at the surfaces of the both portions;
wherein;
where a base curve is represented by BC; a mean surface refracting power at an arbitrary point on the progressive-power multifocal surface, located at a distance of x (mm) from a far-vision portion eyepoint in the horizontal direction in wear of spectacles and located at a distance of y (mm) from the far-vision portion eyepoint in the vertical direction in wear of spectacles, by P (x,y) (diopter); and a mean surface additional refracting power obtained by subtracting the base curve BC from the mean surface refracting power, by xcex94P(x,y){=P(x,y)xe2x88x92BC} (diopter);
the lens fulfills the condition of
xcex94P(x,y) less than 0 xe2x80x83xe2x80x83(1) 
in a region which satisfies 15xe2x89xa6(x2+y2)xc2xdxe2x89xa620 in the far-vision correction portion.
According to a preferred embodiment of the present invention, the lens fulfills the condition of
0.005xe2x89xa6xcex94P(x,y)/(x2+y2)xc2xdxe2x89xa60.120 xe2x80x83xe2x80x83(2) 
in a region which satisfies 15xe2x89xa6(x2+y2)xc2xdxe2x89xa620 in the far-vision correction portion.